The continuing focus of this project is to determine structural changes involved in contraction mechanism and to identify essential steps in the pathway of force generation. It was shown earlier that at low ionic strength and low temperature and essential intermediate crossbridge state immediately preceding force generation is an attached state with low affinity between myosin and actin. New evidence shows that at near physiological conditions active force level is similarly inhibited if the binding of the myosin to actin is inhibited in the low affinity states. The results further confirmed the importance of the low affinity but attached crossbridge states in force production and the existence to a significant extent of such states in the relaxed muscle. High resolution equatorial X-ray studies were carried out to investigate whether there are phase changes in transforming the conformation of the crossbridges from relaxed state to rigor state. Preliminary results suggest that one reflection, [3,0] may undergo a phase change. The difference in the conformation of the relaxed and the rigor crossbridges are probably more pronounced than previously proposed. A new project has been initiated to investigate the role of eh strongly bound crossbridge states in regulation and contraction of muscle by introducing exogenous myosin fragments modified by N-ethylmaleimide into the muscle fiber. The first phase of the project has determined the time course of binding of the myosin fragment to actin. Preliminary mechanical data suggest that the binding of such modified myosin fragment increases the sensitivity of Ca++ regulation and inhibits active force level.